Add ons bookable with all staff

I have 8 staff offering different treatments. I made some add ons so that we can upsell and there isn't the changeover time added between appointments like if the customer booked the appointments individually. 

It means though that while I can appointment type to specific therapists, the add ons are bookable with all therapists, so for example my sports massage therapist got booked for a sports massage then manicure which they do not offer. 

This seems like a pretty basic error, why does this happen on the site?